All About IP

* "Internet Protocol" packets are encapsulated in link-layer packets

* Design ideas:
  * Carry IP packets in arbitrary link layers;
    easily transfer between different link layers
  * Route packets
  * Provide support for interacting with gateways / routers
  * Provide support for higher-level protocols

* Replaces "NCP"

IPv4

* Conceptually like Ethernet
  * Source and dest address
  * Length information
  * Type
  * Payload
  * Checksum

* Several common types
  * UDP
  * TCP
  * ICMP

IPv4 Addressing

* 32 bits written in bytewise "dotted-decimal" e.g.
  127.0.0.1

* Four kinds of bits "left-to-right" (maybe)
  * Address class = how to interpret remaining bits
  * Network part = where the core should send the packet
  * Subnet part = where on the local collection of link-level
                  networks the network should send the packet
  * Host part = which IP interface on the link-level network
                should get the packet

* Allocated by Internet Assigned Numbers Authority (IANA) through
  retail chain involving core providers, ISPs

* We are essentially out of IPv4 addresses
  * However, this has as much to do with router constraints
    as it does with address bits


Addresses For Interfaces

* Given host may have multiple interfaces
  * Link or IP layer
  * Physical or virtual

* No explicit binding between addresses and hosts

* Creates issues around "the host address"

* Issues of address assignment / binding between layers
  deferred for several weeks: ARP, DNS, DHCP etc

Forwarding

* Steps to forward an IP packet
  * Mask off the network and subnet parts
  * If the subnet is yours, forward to the host using
    link layer
    * "Overlaid subnets"
  * If the network is yours, forward toward the subnet
  * Otherwise, forward toward the Internet gateway

* Masks
  * Bitmasks might be dotted-decimal or /#
  * Simply anded with  address to get network / subnet
  * Usually prefixes

Local networks and NAT

* 10.0.0.0/8, 196.168.0.0/16
  * Not routable through the core Internet
  * "Network Address Translation" does source / dest
    address change between single core and multiple local



Fragmentation

* IP Maximum Transmission Unit ("MTU") must be at least
  68 bytes ("path MTU" -- "host MTU" is 576 bytes)

* Each link-level network between source and destination
  may have different MTU

* Want to be able to send "big" packets

* Solution: IP "Fragmentation"
  * Break the big payload into pieces and send them
    separately
  * Use ID and Segment bits in IP header for construction
    and reassembly
  * Arguably should have been end-to-end

IPv6

* Main difference is addresses: 32-bit vs 128-bit
  * Also a bunch of details

* Designed to let IP scale farther

* Somewhat stalled
  * Transition details
  * Router capacity
  * Lack of demand


UDP, TCP, ICMP

* Protocols encapsulated in IP
  * User Datagram Protocol "UDP": Thin layer
    under IP providing "port" abstraction and data
    checksum
  * Transmission Control Protocol "TCP": Provides
    sequenced, reliable streams with out-of-band
    data etc via windowing, acks

  * Internet Control Message Protocol "ICMP": Host-to-router
    and router-to-router messages for "out of band" IP
    activities, e.g. "ICMP echo" AKA "ping"